Toxoplasma gondii is significant pathogenic eukaryote that causes congenital defects, recurrent ocular infections, and devastating opportunistic infections in humans. Toxoplasma is widely dispersed into various hosts in nature due to the remarkable ability of this parasite to transmit to new hosts and to then successfully navigate acute infection and establish chronic infection in many hosts. Work from several labs has clearly established that Toxoplasma secretes proteins from rhoptry (ROPs) and dense granule (GRAs) organelles and several of these proteins are known to manipulate host cells to disarm innate immunity and confer resistance to host interferon-?. Other ROP and GRA proteins clearly modulate the host immune response, particularly IL- 12 and inflammatory cytokines, but individually do not appear to strongly influence parasite virulence or chronic infection. For example, the secreted GRA15 molecule activates NF-?? in low virulence type II strains and this molecule increases production of host IL-12. While the secreted ROP16 molecule of virulent type I or type III strains sustains the activation of transcription factors STAT3 and STAT6 and reduces host IL-12 production, the low virulence type II ROP16 molecule does not activate STAT3 or STAT6 and was previously hypothesized to be inactive. In preliminary data, we show that individual deletion of low virulence type II GRA15 or ROP16 did not affect acute virulence or chronic infection. Surprisingly, however, we discovered that simultaneous deletion of low virulence type II GRA15 and ROP16 abolished acute virulence as well as the ability to establish a chronic infection. These remarkable phenotypes suggest that type II ROP16 together with GRA15 provide a key virulence mechanism for successful parasitism. Our preliminary findings suggest that this virulence mechanism is active in both murine and human hosts that are commonly infected by low virulence type II strains. The goal of this exploratory high impact R21 application is to identify the targets and the mechanism(s) associated with low virulence type II ROP16 and GRA15 molecules that provide resistance against the host to enable successful acute and chronic infection.